2.76 - How to use ATI/AMD GPU without Linux proprietary driver?
I'm affraid that is not possible. Apparently (checked on luxrender) the GPU is only detected by blender using privative drivers, but even knowing that cycles is not fully capable with ATI/AMD cards. The kernel must compiled on the fly before start rendering and you can't use (even with the propietary drivers) volume absorption or scattering with the GPU and you still have to use CPU for it.
Blender works better with Nvidia CUDA i got the experience using blender on linux for several years and comparing with windows. and the results with AMD cards are disapointing.